Monitoring soil quality indices and soybean yield as influenced by integrated biochar and drought stress

Maintaining soil quality is a priority, especially in areas with environmental stresses such as drought. Therefore, application of amendments such as biochar for improving soil quality in these conditions received increased attention. On the one hand, soil quality is a versatile integrated indicator of soil changes in response to different applied treatments (e.g., biochar and drought) and managerial practices. Conversely, the integrated impacts of biochar and drought on plant performance and soil quality indices (SQIs) have been not studied in previous works. Therefore, we aimed to (1) evaluate the integrated impacts of cattle manure-derived biochar (0, 25, 50, and 100 t ha−1) and soil moisture levels (100, 70, and 55% of field capacity, FC) on soybean yield and soil–plant nutrients status; (2) determine quality indices of the Integrated and Nemero SQI in the postharvest soils based on two indicator selection scenarios of the Total (TDS) and Minimum (MDS) Data Set by applying Principal Component Analysis (PCA) to decline soil parameters to the most effective ones in response to the applied levels of biochar and soil moisture conditions; (3) determine the most effective soil attributes to calculate SQIs; and (4) determine the soil quality (SQ) grades under the applied treatments. Selection criterion was also calculated to select the effective properties of principal component which in turn substantially controls changes. According to the results of PCA, from the 26 applied physicochemical properties, seven principal components (PCs) had eigenvalues of greater than one and showed 86.95% of data cumulative variance. Among the 26 studied soil indicators, the most effective ones with the highest importance to soil quality indices which determined as the minimum data set were soluble potassium, mean weight diameter of aggregates obtained by wet sieving and also that of dry sieving, penetration resistance, pH of saturated paste, geometric mean diameter of aggregates measured by dry sieving, and available zinc with the eigenvectors of 0.964, 0.911, 0.968, 0.894, 0.874, 1.009, and 0.702, respectively. The highest positive correlations between soil quality indices and plant nutrients concentration were obtained in soils received 25 t biochar ha−1. The highest Integrated SQI based on the TDS was observed with addition of 100 t biochar ha−1 under 70% and 55% field capacity conditions. Integrated SQI based on the TDS approach increased from grade IV to grade I when high rates of biochar were applied in drought stress conditions. This indicates that biochar has a remarkable capability for increasing the capacity of the soils to retain water as well as for improving SQ which is very important, particularly in semi-arid and arid regions facing water scarcity problems.